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History of the hypothesis and geologic evidence Fig.1: Physicist Luis Alvarez and his geologist son Walter, next to the thin dark clay layer (above Walter’s right hand) sandwiched between limestone beds, outcropping near Gubbio, Italy. Fossil species that are used to distinguish one layer from another are called index fossils.

Image courtesy of Berkeley Lab (CC BY-NC-ND 2.0) The original physical evidence that led Alvarez et al.[1] to suggest the impact hypothesis was an anomalously high content of the heavy element iridium in thin clay deposits stratigraphically placed at the so-called K-Pg boundary (), the contact between Cretaceous and Paleogene rocks.[2] Iridium is much more abundant in meteorites than in the rocks of Earth’s crust.

In the figure, that distinct age range for each fossil species is indicated by the grey arrows underlying the picture of each fossil.

Using the overlapping age ranges of multiple fossils, it is possible to determine the relative age of the fossil species i.